Three years after the first outbreaks of bird flu in Asia, experts are still puzzling at how the disease spread across three continents so quickly and how wild birds have helped disseminate the deadly virus.

More than 300 scientists and animal experts discussed these and other questions at a two-day conference which opened in Rome on Tuesday.

The meeting was organized by the U.N. Food and Agriculture Organization, based in Rome, and by the Paris-based World Organization for Aniabdo

Experts were invited from about 100 countries.

The H5N1 strain of bird flu has killed at least 127 people worldwide and ravaged poultry flocks in Asia, Europe and Africa, but experts are still unsure if migrating birds or the commercial poultry trade deserve most of the blame for spreading the disease.

Also experts wonder why the virus, widespread in South East Asia since 2003, only started moving across the continent to Europe and Africa last year, said Samuel Jutzi, director of FAO´s animal production and health division.

"Why all of a sudden that happened is not entirely clear," Jutzi told The Associated Press on the eve of the conference. "And if the wild birds had a role in that, why didn´t they have one before?"

So far, most human cases have been traced to contact with infected poultry, but experts fear the deadly virus could mutate into a form that passes easily from human to human, possibly sparking a global pandemic. Understanding how the bird flu virus spreads is a key factor in the fight against the disease.

Evidence on the role of wild birds is not always conclusive in the areas where H5N1 has appeared. Migratory birds introduced the disease in Russia and Eastern Europe, but in the case of recent outbreaks in Africa no evidence has yet been found pointing to wild birds, Jutzi said.

"Ornithologists are very knowledgeable on the movement of the birds but not on their diseases," he said. "We hope the conference will indicate some research in this direction."

So far, research shows that wild birds are likely to introduce the virus in unaffected areas but that the disease becomes widespread mostly through poor hygiene and through poultry trade, Jutzi said.